In the formation of semiconductor devices, and specifically in the fabrication of semiconductor integrated circuits, an oxide layer, usually called a "field oxide," is used to isolate adjacent active devices. This is especially true with metal-oxide-semiconductor field effect transistor (MOSFET) circuits but is true of bipolar and other circuits as well.
Device isolation in the deep submicron region is one of the major obstacles faced by high density complementary metal oxide semiconductor (CMOS) technologies. Traditionally local oxidation of silicon (LOCOS) has been employed to electrically isolate active devices in the CMOS technologies. In the LOCOS process, field oxide is usually formed by a process in which an oxidation resistant material such as silicon nitride or a combination of silicon nitride with silicon oxide or other materials is formed on a semiconductor substrate overlying active device regions where transistors or other devices are to be formed. The substrate is then heated in an oxidizing ambient to grow a thermal oxide on those portions of the substrate not protected by the oxidation resistant material. Depending upon the oxide thickness a portion of the unprotected silicon is consumed during the formation of the thermal oxide.
There are a number of problems associated with the use of LOCOS. As the minimum feature sizes get below about 0.4 micrometers LOCOS can no longer be applied primarily due to encroachment concerns, which are typically 0.2 micrometers per side, and the related effects, including narrow-width effects which affect device performance. These problems are especially significant with newer circuits which utilize an ever increasing number of devices and in which the devices are of ever decreasing size, both in surface area and depth. As thermal oxides grow in thickness, they also grow laterally under the edge of the oxidation resistant material causing a lifting of the edge of the oxidation resistant material, and giving rise to what is known as "bird's beak." Thick field oxides thus use a considerable amount of lateral space and accordingly require an increase in the size of the circuit chip. This is especially true when the circuit involves a high density of integration. This "lateral encroachment" or "birds beak" also generates large stresses in the oxidation resistant material near the active area edges or corners, which can create dislocations in the active area regions resulting in higher junction leakage. These stress related defects, therefore, result in poor functionality when LOCOS is employed in addition to encroachment problems. Various modifications to LOCOS which reduce the lateral encroachment have been proposed. Most of these techniques are complicated and hence add expense to the process flow. In addition to LOCOS other isolation approaches include trench and isolation by selective epitaxy. Articles by B. Davari et al., A VARIABLE-SIZE SHALLOW TRENCH ISOLATION (STI) TECHNOLOGY WITH DIFFUSED SIDEWALL DOPING FOR SUBMICRON CMOS, p. 92, IEDM 1988, which refers to the trench, and by Y. C. S. Yu et al., NEW BIRD'S BEAK-FREE DEVICE ISOLATION TECHNOLOGY, J. Electrochem. Soc, 135, p. 2562, 1988, which refers to epitaxy, are incorporated by reference to familiarize the reader with one level of knowledge of one skilled in the art.
Oxygen implants are used extensively in separation by implantation of oxygen (SIMOX) implementation of silicon-on-insulator (SOI) technology. These oxygen implants are carried out by implanting oxygen ions at high energy and doses into bare silicon wafers. A buried oxygen rich layer is formed, and extensive damage occurs. The oxygen is chemically bonded to silicon to form silicon oxide during high temperature annealing in nitrogen. This anneal step is also used to anneal out the damage created by the oxygen implantation. Investigators in the SOI area have also investigated low energy oxygen implants to create an oxide layer close to the Si surface. This method is described by J. Dylewski and M. C. Joshi in FORMATION OF THIN SiO.sub.2 FILMS BY HIGH DOSE OXYGEN ION IMPLANTATION INTO SILICON AND THEIR INVESTIGATION BY IR TECHNIQUES, Thin Solid Films, v. 35, p. 327 (1976) and by K. I. Kirov, et al., PROPERTIES OF SiO.sub.2 FILMS FORMED BY OXYGEN IMPLANTATION INTO SILICON, Thin Solid Films, v. 48, p. 187 (1978), which are herein incorporated by reference to familiarize the reader with one level of knowledge of one skilled in the art. It is shown in these articles that using a low energy ion implant can create a buried oxygen rich layer near the surface of the substrate.
In one LOCOS modification, described in patent 4,748,134, entitled ISOLATION PROCESS FOR SEMICONDUCTOR DEVICES, by Holland et al., halogen ions are implanted in order to enhance the oxide growth rate in the vertical direction compared to the lateral direction. In some experiments conducted by the inventors of U.S. Pat. No. 4,748,134 for sequential energies and doses of 160 keV/1.times.10.sup.16 /cm.sup.2, 85 keV/6.4.times.10.sup.15 /cm.sup.2, and 40 keV/3.5 .times.10.sup.15 /cm.sup.2, it was found that implanting oxygen ions into the substrate resulted in "only a negligible increase in thickness" of the field oxide, see Example 1 which starts on line 16 in column 4. The Claims of U.S. Pat. No. 4,748,134 are directed to halogen implants for increasing the growth rate of oxide in a vertical direction relative to the growth rate in a lateral direction.
Because of the foregoing and other problems associated with the conventional formation of isolating thermal oxides, there exists a need for an improved process.